Astrocytes and microglia but not neurons preferentially generate N-terminally truncated Aβ peptides

Abstract

The neuropathological hallmarks of Alzheimer's disease include extracellular neuritic plaques and neurofibrillary tangles. The neuritic plaques contain β-amyloid peptides (Aβ peptides) as the major proteinaceous constituent and are surrounded by activated microglia and astrocytes as well as dystrophic neurites. N-terminally truncated forms of Aβ peptides are highly prevalent in neuritic plaques, including Aβ 3-x beginning at Glu eventually modified to pyroglutamate (Aβ N3pE-x), Aβ 2-x, Aβ 4-x, and Aβ 5-x. The precise origin of the different N-terminally modified Aβ peptides currently remains unknown. To assess the contribution of specific cell types to the formation of different N-terminally truncated Aβ peptides, supernatants from serum-free primary cell cultures of chicken neurons, astrocytes, and microglia, as well as human astrocytes, were analyzed by Aβ-ELISA and one- and two-dimensional SDS-urea polyacrylamide gel electrophoresis followed by immunoblot analysis. To evaluate the contribution of β- and γ-secretase to the generation of N-terminally modified Aβ, cultured astrocytes were treated with membrane-anchored "tripartite β-secretase (BACE1) inhibitors" and the γ-secretase inhibitor DAPT. Neurons, astrocytes, and microglia each exhibited cell type-specific patterns of secreted Aβ peptides. Neurons predominantly secreted Aβ peptides that begin at Asp1, whereas those released from astrocytes and microglia included high proportions of N-terminally modified Aβ peptides, presumably including Aβ 2/3-x and 4/5-x. The inhibition of BACE1 reduced the amount of Aβ 1-x in cell culture supernatants but not the amount of Aβ 2-x.

Cholesterol‐induced astrocyte activation is associated with increased amyloid precursor protein expression and processing

“…However, the astrocyte clearance function can also be compromised in AD (Mulder et al, 2012), and a significant increase in secreted Ab might occur in this pathological situation. However, the small changes detected in Ab40/ 42 could be because astrocytes are able to produce and secrete different patterns of N-terminally truncated Ab variants that were no detected in this study (Oberstein et al, 2014). Finally, it is possible that in the present model, cholesterol promoted the interaction between BACE-1 and APP without affecting the activity of g-secretase to complete the Ab production.…”

Solid‐Phase Synthesis and Characterization of N‐Terminally Elongated Aβ−3–x‐Peptides

“…Six years ago, we reported that treatment of SH‐SY5Y cells overexpressing wild type human APP with membrane‐anchored tripartite BACE1 inhibitors reduced the overall cellular generation of Aβ peptide, but at the same time led to a relative increase in specific Aβ peptide variants displaying isoelectric points (pI) different from that of known Aβ peptides starting with Asp(1), that is, pI of approximately 5.4 according to 2D‐Western blot analysis . Mass spectrometry data indicated that the BACE1‐inhibitor‐resistant Aβ peptides comprise primarily N‐truncated peptides starting with Arg(5), and have a pI of approximately 6.5 according to a novel capillary isoelectric‐focusing (CIEF) immunoassay . In addition, a minor fraction of BACE1‐inhibitor‐resistant Aβ peptides with a pI of approximately 6.0 was observed in supernatants of APP overexpressing SH‐SY5Y cells (see Figure 2 in ref.…”

“…[32] Mass spectrometry data indicated that the BACE1-inhibitor-resistant Ab peptides comprise primarily N-truncated peptides starting with Arg(5), and have ap Io fa pproximately 6.5 according to an ovel capillary isoelectric-focusing (CIEF) immunoassay. [16,18,[33][34][35] In addition, am inor fraction of BACE1-inhibitor-resistant Ab peptides with ap Io fa pproximately 6.0 was observed in supernatants of APP overexpressing SH-SY5Yc ells (see Figure 2i nr ef. [18]).…”

Glial contributions to neuronal health and disease: new insights from Drosophila

“…The fly genome contains one APP-related gene, APP-like (APPL), which, when mutated, causes defects in neuronal outgrowth, synaptic stability, and behavior, and Appl mutant flies have provided important insight into endogenous APP signaling mechanisms in the CNS (46). Across all model organisms, an overwhelming number of studies have explored the role of neuronal APP, but relatively little is known about the function of glial APP, despite the fact that APP is also expressed in mammalian oligodendrocytes, astrocytes, and microglia (21, 47). A recent study explored how glial APPL influences sleep-wake cycles in adult flies.…”